Immutable.js returns new reference after each operation on its collection. When you have Redux state that is immutable this might be a problem and it might cause unwanted re-renders. All Immutable.js collections have a function called hashCode() that returns same string as long as the collection contain the same elements in same order. You can use that to create a memoization function as follows.

const memoizeByHashCode = () => {
  let oldValue = {};
  return (value: Object) => {
    if (Object.keys(oldValue).length === 0) {
      oldValue = value;
      return value;
    } else if (value.hashCode() === oldValue.hashCode()) {
      return oldValue;
    } else {
      oldValue = value;
      return value;

Now you can use this function to return same reference if value did not change

const remember = memoizeByHashCode();
const collection1 = remember(add(1));
const collection2 = remember(add(1));

Now collection1 and collection2 will point to the same reference.

This post is actually cross published from–Memoization-of-Immutable.js-using-hash-code/

This post is also available on DEV.